System and method for measuring subsurface temperature of tissue during ablation

ABSTRACT

An RF ablation system comprises irrigated split tip electrode catheter, an RF generator and a signal processor. The catheter comprises four orthogonally arranged electrodes at the distal tip. The catheter is used to map the electrical activity of a heart chamber to locate site(s) of aberrant electrical pathways to be ablated. Once an ablation site has been located, the signal processor activates the RF generator to transmit a low level RF current to each electrode member of the split tip electrode. The signal processor receives signals indicative of the impedance between each electrode member and one or more surface indifferent electrodes and determines which electrode members are associated with the highest impedance. Such electrode members are those in greatest contact with the myocardium. The signal processor then automatically activates the RF generator to transmit an RF ablation current to the electrode members in contact with the myocardium to create a lesion.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. Pat. application No.09/205,628, filed Dec. 3, 1998.

FIELD OF THE INVENTION

This invention relates to an electrophysiology catheter system having asplit tip electrode catheter and a signal processing system forproviding a safe and effective RF ablation of the heart tissue.

BACKGROUND OF THE INVENTION

The heart has a natural pacemaker and conduction system which causes theheart muscle to contract, or beat, in an orderly rhythmical manner. Thenormal pacing rate for an adult at rest is about 60 to 70 beats perminute. There are many physiologic abnormalities which cause one or morechambers of the heart to beat more rapidly (tachycardia or flutter) orchaotically (fibrillation). A patient cannot live with ventricularfibrillation because there would be no blood pumped through thearteries, but may live with atrial fibrillation so long as the chaoticimpulses are filtered out at the AV node and do not reach theventricals. A patient may also live with atrial flutter and variousforms of tachycardia but quality of life may be considerablycompromised.

Many of these arrhythmias can be treated effectively by ablation usingradio-frequency (RF) energy. Other arrhythmias are less effectivelytreated, requiring more RF lesions for a successful outcome or resultingin no successful outcome. RF ablation is performed with a catheterhaving one or more electrodes which deliver the RF energy to the cardiactissue. In operation the catheter is guided through a vein or arteryinto the heart chamber and positioned at one or more sites, determinedby an electrophysiologist, to correct the arrhythmia. The catheterdelivers energy from an external source (generator) to the tissue,generating sufficient heat to kill the tissue, which is thereafterreplaced by scar tissue. In a successful ablation procedure, the lesionsformed interrupt the electrical pathways that cause the arrhythmia sothat heart rhythm is improved or returns to normal.

During ablation, it is important to control the temperature of thetissue, both at the tissue surface and below. If the surface temperaturebecomes excessive, dehydration and charring. results. Charred tissuepresents a dangerous situation as it may flake off resulting in blockageof a blood vessel. Excessive heating below the surface is also dangerousas it may result in a “steam pop.” A “steam pop” occurs when deep tissueis heated to a temperature sufficient to boil the water of the tissuewhich creates a steam bubble within the tissue. Such a steam bubbleerupts through the surface of the myocardium with substantial force.This eruption can typically be heard as a “pop” by theelectrocardiologist.

SUMMARY OF THE INVENTION

The present invention provides an improved system and method forablating myocardial tissue. The system comprises an electrophysiologycatheter having a split tip electrode at its distal end. The split tipelectrode preferably comprises a tip electrode assembly having two ormore orthogonally arranged electrode members. In a preferred embodiment,there are five electrode members which comprise the split tip electrode.The electrode members are arranged so that four of the members formsectors of a hemisphere and the fifth forms a ring behind the fourmember hemisphere. The split tip electrode catheter preferably comprisesmeans for passing a fluid, e.g., saline, through each of the electrodemembers for cooling the electrode members during ablation. Aparticularly preferred irrigated split tip electrode is disclosed inpatent application entitled IRRIGATED SPLIT TIP ELECTRODE CATHETER toWebster, Jr. (U.S. application No. 09/205,116) which is filedconcurrently herewith and incorporated herein by reference.

The system further comprises means, electrically connected to each ofthe electrode members of the split tip electrode, for receivingelectrical signals from each of the electrode members and for generatinga record and/or display indicative of those signals, preferably anelectrogram.

Also electrically connected to each of the electrode members of thesplit tip electrode is a means for measuring the electrical impedancebetween each electrode member and a reference electrode to determinewhich of the electrode members are in contact with the myocardium. Theimpedance measuring means comprises an RF generator for generating a lowlevel RF electrical signal, preferably about 2 microamperes at afrequency of about 50 KHz, means for delivering the low level RF currentto each of the electrode members, at least one reference or indifferentelectrode, e.g., a skin patch electrode, and preferably means forgenerating a record and/or display of the impedance between eachelectrode member and the reference electrode(s).

Means are also provided for delivering an ablating RF current to one ormore of the electrode members of the split tip electrode for ablatingmyocardial tissue in contact with those electrode members. Such meanscomprises an RF generator for generating RF current sufficiently strongto ablate heart tissue. Preferred ablating currents are from about 0.25to about 1.0 amperes at about 400 KHz to about 700 KHz, more preferablyabout 0.5 to 0.75 amperes at 500 KHz. A signal processor is provided foractivating the RF generator to transmit the low level RF current to eachof the electrode members of the split tip electrode and for comparingthe impedances associated with each electrode member of the split tipelectrode to determine which electrode member(s) is (are) associatedwith the highest impedance. This allows the identification of theelectrode members in contact with the myocardium as those electrodemembers will be associated with a higher impedance than those electrodemembers in contact only with the blood pool. The signal processor alsoactivates the RF generator to selectively transmit RF ablation currentto only those electrode members in contact with the myocardium.

It is preferred that the system comprise an irrigated split tipelectrode catheter and a metering pump for pumping a cooling fluidthrough the catheter to cool the ablating electrode(s) during ablation.In such an embodiment, it is also preferred that the signal processor becapable of activating the pump and the RF generator intermittently sothat there are periods of no irrigation and RF ablation energy betweenperiods wherein RF ablation energy and cooling fluid are delivered tothe ablation electrodes.

In a preferred embodiment of the invention, the ablation system furthercomprises an irrigated split tip electrode catheter and means formonitoring the surface and/or subsurface and temperatures of the tissuebeing ablated. Preferred surface temperature monitoring means comprisesa thermocouple or thermistor coupled to each electrode member of thesplit tip electrode for generating an electrical signal indicative ofthe temperature of the electrode members in contact with the myocardiumand signal processor intermittently activating and deactivating the RFablation current generator and the irrigation pump. During thedeactivated periods, the electrode temperature reaches the approximatesurface temperature of the tissue being ablated and is therefore anestimate of the tissue temperature at the tissue-electrode interface.Preferred sub-surface temperature monitoring means comprises impedancemonitoring means for monitoring the impedance associated with theelectrode members of the split tip electrode, and estimating the maximumdeep tissue temperature from the impedance measurements. Preferably,means are also provided for automatically reducing the amount of RFcurrent delivered to the selected electrode members when the temperatureand/or impedance reaches pre-determined levels to prevent excessivesurface or deep tissue temperature rise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing preferred irrigated split tipelectrode catheter and signal processing RF ablation system.

FIG. 2 is a transverse cross-sectional view of the catheter body alongline 2—2 in FIG. 1.

FIG. 3 is a perspective view of the tip section of the preferredirrigated split tip electrode catheter of FIG. 1.

FIG. 4 is an exploded perspective view of the tip section of thepreferred irrigated split tip electrode of FIG. 1.

FIG. 5 is a side view of the split tip electrode of the catheter of FIG.1.

FIG. 6 is a cut-away perspective view of the proximal portion of thecomposite electrode of the catheter of FIG. 1 showing the connection ofthe electrode lead and safety wires.

FIG. 7 is a longitudinal cross-sectional view of the catheter tipsection of the embodiment of FIGS. 1 to 6 in which the infusion tubecomprises two sections.

FIG. 8 is a longitudinal cross-sectional view of a tip section of apreferred embodiment of the invention which does not comprise a bridgetubing;

FIG. 8A is a longitudinal cross-sectional view of a portion of the tipsection of FIG. 8 showing a preferred means for anchoring puller wiresto the side wall of the tubing of the tip section.

FIG. 9 is a longitudinal cross-sectional view of the tip section of apreferred embodiment of the invention in which the split tip electrodedoes not include a cup electrode.

FIG. 10 is a longitudinal cross-sectional view of the tip section ofanother embodiment of the invention similar to FIG. 9, in which thesplit tip electrode includes a ring electrode.

FIG. 11 is a longitudinal cross-sectional view of the tip section ofanother preferred embodiment of the invention including anelectromagnetic sensor.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, there is shown a schematic diagram of apreferred system for mapping electrical signals in the heart to locateaberrant electrical pathways and to ablate heart tissue to interruptsuch aberrant electrical pathways. The system comprises a split tipelectrode catheter 10, preferably an irrigated split tip electrodecatheter, as shown, one or more reference or indifferent electrodes 5, asignal processor 6, an RF current generating unit 7, a monitor 8 and ordisplay, and a pump 9 for pumping a fluid into and through an infusiontube in the catheter.

The catheter 10 comprises an elongated catheter body 11 and a tipsection 12 at the distal end of the catheter body 11 and a controlhandle 13 at the proximal end of the catheter body 11. The catheter body11 comprises an elongated tubular construction having a single, centralor axial lumen 25. The catheter body 11 is flexible, i.e., bendable, butsubstantially non-compressible along its length. The catheter body 11can be of any suitable construction and made of any suitable material. Apresently preferred construction comprises an outer wall 15 made of apolyurethane or the like and an inner stiffening tube 19. The outer wallcomprises an imbedded braided mesh of stainless steel or the like toincrease torsional stiffness of the catheter body 11 so that, when thecontrol handle 13 is rotated, the tip section of the catheter 10 willrotate in a corresponding manner. The outer diameter of the catheterbody 11 is not critical, but is preferably no more than about 8 french.Likewise the thickness of the outer wall is not critical.

The inner surface of the outer wall is lined with a stiffening tube 19,which can be made of any suitable material, preferably polyimide. Thestiffening tube 19, along with the braided outer wall 15, providesimproved torsional stability while at the same time minimizing the wallthickness of the catheter, thus maximizing the diameter of the singlelumen. The outer diameter of the stiffening tube 19 is about the same asor slightly smaller than the inner diameter of the outer wall. Polyimidetubing is presently preferred because it may be very thin walled whilestill providing very good stiffness. This maximizes the diameter of thecentral lumen without sacrificing strength and stiffness.

A particularly preferred catheter body 11 has an outer wall with anouter diameter of about 0.092 inch and an inner diameter of about 0.063inch and a polyimide stiffening tube having an outer diameter of about0.061 inch and an inner diameter of about 0.052 inch.

As shown in FIG. 2, extending through the lumen are an infusion tube 14,a plurality of electrode lead wires 16, having an insulation coating,and a compression coil 17 in surrounding relation to a puller wire 18.

An enlarged side view of the tip section 12 is shown in FIGS. 3 and 4.The tip section comprises a split tip electrode 20 comprising a “quad”tip electrode, i.e. a four member tip electrode assembly 30 and a cupelectrode 36. The tip section 12 further comprises a bridge tubing 22and a section of flexible tubing 24. The tubing 24 is made of a suitablenon-toxic material which is preferably more flexible than the catheterbody 11. A presently preferred material for the tubing 24 is braidedpolyurethane, i.e., polyurethane with an embedded mesh of braidedstainless steel or the like. The outer diameter of the tip section 12,like that of the catheter body 11, is preferably no greater than about 8french. The proximal end of the flexible tubing 24 is attached to thedistal end of the catheter body 11 by any suitable means.

In the embodiment shown, the flexible tubing 24 has three off axislumens, a first lumen 26 through which the infusion tube 14 extends, asecond lumen 27 through which the electrode lead wires 16 extend and athird lumen 28 through which the puller wire 18 extends. The diameter ofthe three lumens may be the same or may differ a desired. The presentlypreferred diameters of the first, second and third lumens are about0.035 inch, about 0.022 inch and about 0.022 inch. The length of theflexible tubing is not critical but is preferably about 2 to 3 inches.The distal end of the flexible tubing is recessed or stepped down toform a distal stem 29 which fits into the proximal end of the bridgetubing 22.

With reference to FIG. 4, the quad tip electrode 30 has a distal portion31 having an exterior surface with a preferably rounded or bullet shapeddistal end, and a proximal portion 32 which forms a recessed stem. Theouter diameter of the distal portion 31 is preferably 8 french or less.The overall length of the distal portion is not critical but issufficient for ablation. A presently preferred length is about 2.5 mm.

The quad tip electrode 30 comprises four electrode members 33. Eachelectrode member 33 comprises a pair of flat interior surfaces and anexterior surface. The electrode members 33 are arranged with each flatside of each electrode member adjacent to a flat side of anotherelectrode member, separated therefrom by insulation 34, preferablyhigh-temperature epoxy. To facilitate the connection of electrode andlead wires 16, each electrode member 33 further comprises an electrodelead bore 42 in its proximal end generally parallel to the axis of thecatheter body 11.

Each of the electrode members 33 can be of any suitable construction andis preferably made of platinum. It is understood that the number, shapeand various dimensions of the electrode members 33 are not critical andmay be varied as desired.

The cup electrode 36 is hollow and comprises a generally cylindricalside wall 37 and a generally flat proximal end wall 38. The distalportion of the side 37 wall has an exterior surface 39 forming a ringelectrode. The proximal portion of the side wall 37 is recessed. Thestem 32 of the quad tip electrode 30 is received and secured within thehollow interior of the cup electrode 36, by polyurethane glue or thelike which electrically insulates the composite tip electrode 30 fromthe cup electrode 36.

The quad tip electrode 30 comprises a central irrigation channel 40,formed for example by drilling, for receiving the distal end of theinfusion tube 14. The central irrigation channel 40 extends axially fromthe proximal end of the quad tip electrode 30 to about the midpoint ofthe quad tip electrode 30. There, the channel 40 divides into fourgenerally transverse branches 41, each branch extending distally andradially, e.g., at a 45° angle, through a separate electrode member 33.The diameters of the central channel 40 and branches 41 are notcritical. A presently preferred composite tip electrode has a centralirrigation channel 40 having a diameter of about 0.5 mm and fourbranches 41, each having a diameter of about 0.4 mm.

It is to be understood that the size and number of channels and/orbranches may vary as desired. For example, each electrode member mayhave a plurality of branches rather than a single branch. Each branchmay comprise secondary branches if desired. Rather than definedbranches, the electrode members may be made of a porous material, e.g.as described in U.S. Pat. Nos. 5,643,197, and 5,462,521, which areincorporated herein by reference.

If desired, less than all of the electrode members may have irrigationbranches or channels. For example, if only one or two of the electrodemembers 33 are intended for delivering RF energy during an ablationprocedure, it may be desired that only those electrode members 33comprise irrigation branches or channels.

The cup electrode 36 comprises a plurality of pass-through bores 54 forallowing insulated electrode lead wires 16 for each of the electrodemembers 33 of the quad tip electrode 30 to pass through the proximal endwall 38 of the cup electrode 36 when the electrode catheter is fullyassembled. Each of the pass-through bores 54 are generally parallel tothe axis of the catheter body 11, are aligned with an associatedelectrode lead bore 42, and are equally spaced about the proximal end ofthe cup electrode 36. To further insulate the cup electrode 36 from thequad tip electrode 30, each of the pass-through bores 54 has a meniscusinsulator with a hole defined therethrough, as shown and described inU.S. Pat. No. 5,836,875 to Webster, Jr., which is incorporated herein byreference. The meniscus insulator precludes the electrical contact ofthe associated electrode lead wire 16 with the cup electrode 36.

In the embodiment shown, the cup electrode 36 further comprises an axialhole with a proximally extending cylindrical flange 51 around the hole.The distal portion of the infusion tube 14 extends through the flange 51and axial hole and into the central irrigation channel 40 of the quadtip electrode 30 where it is fixedly secured by polyurethane glue or thelike.

The flange 51 has a transverse hole 53 at about its mid-point. Anelectrode lead wire 16 and a pair of safety wires 56 are fixedlyattached to the stem 51 of the cup electrode 36. A preferred method ofattaching the electrode lead wire 16 and safety wires 56 is shown inFIG. 6. The electrode lead wire 16 and safety wires 56 are inserted intothe flange 51, passed out through the hole 53 and wrapped, preferably1-½ times, around the flange 51. They are then soldered into place. Theproximal end of the safety wires may be secured e.g. by polyurethaneglue or the like anywhere in the tip section 12 proximal to the bridgetubing 22.

The cup electrode 36 can be of any suitable construction and ispreferably made of platinum. The dimensions of the cup electrode are notcritical. In a presently preferred embodiment, the length of the cupelectrode 33 is about 0.13 inch, the depth of the cavity 50 about 0.08inch, the outer diameter about 0.09 inch with a cavity diameter of about0.08 inches.

The bridge tubing 22 is made of a short section of rigid tubularplastic, preferably PEEK (polyetheretherketone), and has an outerdiameter about the same as flexible tubing 24 of the tip section 12 andan inner diameter about the same as the recessed proximal portion of thecup electrode 36 and the recessed distal potion of the flexible tubing24. The bridge tubing 22 connects the split tip electrode 20 to theflexible tubing 24. At its distal end, the bridge tubing 22 receives therecessed proximal portion of the cup electrode 36 which is securedtherein by polyurethane glue or the like. At its proximal end, thebridge tubing 22 receives the recessed distal end of the flexible tubing24 which is also secured therein by polyurethane glue or the like.

The length of the bridge tubing 22 is selected to provide a gap withinthe interior of the bridge tubing 22 between the proximal end of the cupelectrode 36 and the distal end of the flexible tubing 24. The gap issufficiently long to allow space for the infusion tube 14 to bend orcurve from the off-axis lumen 26 in the flexible tubing 24 intoalignment with the axial hole and flange 51 in the cup electrode 33. Theelectrode lead wires 16 extend out of off-axis lumen 27 in the flexibletubing 24 to separate pass-through bores 54 in the cup electrode 36 andto the flange 51 of the cup electrode 33. A bridge tubing 22 having alength of about 6 to about 7 mm and providing a gap of about 1 to about2 mm is presently preferred. It is understood that the bridge tubing maybe made of any suitable,. generally rigid plastic which can withstandthe temperature reached during an ablation procedure withoutsignificant, i.e., detrimental, deformation.

The infusion tube 14 may be made of any suitable material. Polyimide ispresently preferred. It may be a single elongated tube which extendsthrough the catheter body II through the first lumen 26 of the tipsection 12, though the bridge tubing 22 and cup electrode 36 and intothe irrigation channel 40 of the quad tip electrode. Alternatively, asshown in FIG. 7, the infusion tube 14 may comprise two sections, aproximal section which extends through the catheter body 11 and into theproximal end of the first lumen 26 of the tip section, the distal end ofthe proximal infusion tube section being secured in the first lumen 26by polyurethane glue or the like. A second section of the infusion tubeextends from the distal end of the first lumen 26 of the tip section 12,where it is secured by polyurethane glue or the like, through the bridgetubing 22 and flange 51 of the cup electrode 36 and into the centralirrigation channel 40 of the quad tip electrode.

The proximal end of the infusion tube 14 extends out of a sealed openingin the side wall of the catheter body and terminates in a Luer hub orthe like. Alternatively, the infusion tube 14 may extend through thecontrol handle 13 and terminate in a luer hub or the like at a locationproximal to the handle. In either such arrangement, fluids, e.g.,saline, may be introduced into and passed through the infusion tube 14and into and through the electrode members 33 of the composite tip 30 tocool the electrode members 33 during an ablation procedure. It isunderstood that other fluids, e.g., drugs, may also be passed throughthe infusion tube and out composite tip if desired. In a particularlypreferred embodiment, the infusion tube is made out of thin walledpolyamide tubing. It is understood that any suitable material may beused. Preferably having an outer diameter about the same as or slightlysmaller than the diameter of the first lumen 26 of the tip section 12.

In a preferred embodiment, the electrode lead wires 16 associated witheach electrode member 33 of the composite tip electrode 36 and cupelectrode 36 is one wire of a pair of wires of dissimilar metals. Thepresently preferred wire pair is an enameled copper/constantan wire paircomprising a copper wire, having a thickness of about 0.003″ and aconstantan wire, having a thickness of about 0.003″, enameled to thecopper wire. Such an enameled wire pair is described in U.S. patentapplication Ser. No. 08/742,352 to Webster, Jr. which is incorporatedherein by reference. In this configuration, the constantan wire, whichhas high strength supports the copper wire which is soft and fragile.Because the leads are constructed out of two different types of wire,the leads also serve as a thermocouple for measuring the temperature ofthe electrode. It is understood that any temperature monitoring means,e.g., a thermistor, a wire pair used exclusively as a thermocouple, maybe used as desired. The leads 16 may also be used to interrupt powerdelivery in case of irrigation failure. The leads 16 extend into theelectrode lead bores 42 of the electrode members 33 and are fixedlyattached thereto by any suitable means, e.g., soldering or welding.

If desired, the irrigated split tip catheter of the present inventionmay comprise one or more ring electrodes 58 as shown, for example inFIG. 3. Attachment of electrode lead wires to such ring electrodes maybe accomplished by any suitable means and/or procedure. A presentlypreferred procedure for attaching electrode leads to a ring electrode isdescribed in U.S. Pat. No. 5,893,885 to Webster, Jr.

The puller wire 18 extends from the control handle 13 through the lumen20 of the catheter body 11 and into the third off axis lumen 28 of theflexible tubing 24 of the tip section 12. The puller wire 18 is made ofany suitable metal, such as stainless steel or Nitinol, and ispreferably coated with Teflon® or the like. The coating impartslubricity to the puller wire 18. The puller wire 18 preferably has adiameter ranging from about 0.006 to about 0.010 inches. The distal endof the puller wire 18 is anchored at or about the distal end of theflexible tubing 24 by any applicable means. A presently preferred meansfor anchoring the distal end of the puller wire is now described.Briefly, the puller wire 18 comprises a T-bar anchor which is fixedlyattached to the distal end of the puller wire. The crossbar of the T-baranchor lies outside of the distal end of the third off-axis lumen of theflexible tubing or in a notch created in the side wall of the flexibletubing which communicates with the third lumen. The size of the crossbaris selected so that it cannot be pulled into the third lumen. In thisarrangement pulling on the puller wire results in deflection of theflexible tubing of the tip section in the direction of the third lumen.It is understood that the T-bar anchor may be secured in the notch inthe side wall of the tip section tubing, if desired. Alternatively, thepuller wire may be soldered or welded to the cup electrode.

To prevent deflection of the catheter body when the puller wire ispulled, there is provided a compression coil 17 in surrounding relationto the portion of the puller wire extending through the catheter body.The compression coil 17 extends from the proximal end of the catheterbody 11 to the distal end of the catheter body 19 or the proximal end ofthe tip section 12. The compression coil 17 may be made from anysuitable material, but is preferably made from stainless steel. Thecompression coil 17 is tightly wound on itself to provide flexibility,i.e., bending, but to resist compression. The inner diameter of thecompression coil is slightly larger than the diameter of the pullerwire. For example, when the puller wire 18 has a diameter of about 0.007inches, the compression coil 17 preferably has an inner diameter ofabout 0.008 inches. The Teflon ® coating of the puller wire 18 allows itto slide freely within the compression coil 17. Along its length, theouter surface of the compression coil is covered by a flexible,nonconductive sheath to prevent contact between the compression coil 17and any of the lead wires 16. A nonconductive sheath made of polyimidetubing is presently preferred. Such an arrangement involving acompression coil in surrounding relation to a puller wire is describedin U.S. Pat. No. 5,935,124 which is incorporated fully herein byreference.

The compression coil 17 is preferably anchored at its proximal end tothe proximal end of the stiffening tube in the catheter body 11 by aglue joint and at its distal end to the distal and of the catheter body11 or the proximal end of the tip section 12, by another glue joint.Both glue joints are preferably comprised of polyurethane glue or thelike.

Longitudinal movement of the puller wire is controlled by control handle13. The control handle may be of any suitable design. A presentlypreferred control handle for a single puller wire is disclosed in U.S.Pat. No. Re 34,502 to Webster, Jr. which is incorporated herein byreference. Such a handle is particularly applicable if the proximal endof the infusion tube extends out of the catheter body terminating in aLuer hub or the like.

In another embodiment of the invention as shown in FIG. 8 and FIG. 8A,the tip section 12 comprises split tip electrode 20 which is attacheddirectly to the flexible tubing 24. That is, there is no bridge tubing.In this embodiment, the split tip electrode 20 comprises a quad tipelectrode as described above. The cup electrode 36 comprises acylindrical side wall having an exterior surface forming a ringelectrode. However, in this embodiment, there is no recessed proximalportion of the sidewall. That is, the entire sidewall of the cupelectrode 36 forms a ring electrode.

The flexible tubing 24 comprises five lumens, 4 symmetrically spacedoff-axis lumens 61 and an axial lumen 62. The flexible tubing 24comprises an axial bore 63 at its distal end having a diameter slightlylarger than the outer diameter of the cylindrical flange 51 of the cupelectrode 36 to accommodate the safety wires 56 and cup electrode leadwire 16 which is wrapped around the outer circumference of thecylindrical flange 51. If desired, the distal ends of the safety wires56 may be flattened to reduce the diameter of the trepanned hole 63 inthe flexible tubing 24 needed to accommodate the cylindrical flange 51and safety wires 56 wrapped thereabout.

The infusion tube 14 extends through the axial lumen 62 of the flexibletubing 24, through the flange 51 of the cup electrode 36 and directlyinto the central irrigation channel 40 of the quad tip electrode. Asdescribed above, the infusion tube may be a single elongated tube or maycomprise discrete proximal and distal sections.

The electrode lead wires 16, preferably enameled wire pairs as describedabove, for the electrode members 33 of the quad tip electrode extendsthrough one (or more) of the off-axis lumens 62. A small gap 64 isprovided between the distal end of the flexible tubing 24 and theproximal end plate of the cup electrode 36. Within this gap 64, theelectrode lead wires 16 which are not aligned with a pass through borein the cup electrode 36 may pass from the off-axis lumen(s) 61 to passthrough bores 54 of the cup electrode 36 and then into the electrodelead bores 42 of the electrode members 33 of the quad tip electrode 30where they are fixedly attached.

In this embodiment, there are two puller wires 18 which extend throughthe catheter body 11 and into diametrically opposed off-axis lumens 61in the flexible tubing 24. The distal ends of the puller wires 18preferably comprise T-bar anchors 65 as described above and arepreferably anchored by glue 66 in notches 67 in the side wall of theflexible tubing 24 as shown in FIG. 8A. Alternatively, the distal endsof the puller wires may be soldered or welded directly to the proximalend plate of the cup electrode 36.

The cup electrode 36 is fixedly secured to the distal end of theflexible tubing 24 by polyurethane glue or the like which fills the gap64.

In yet another embodiment of the invention shown in FIG. 9, the tipsection comprises a section of flexible tubing 24, a bridge tubing 22and the split tip electrode 20 comprises a quad tip electrode, but nocup electrode. In such an embodiment, the recessed stem 32 of the quadtip electrode extends directly into the distal end of the bridge tubing22 and is secured therein by polyurethane glue or the like.

At its proximal end, the bridge tubing 22 receives the recessed distalend of the flexible tubing 24. The flexible tubing 24 preferablycomprises three off-axis lumens, as described with respect to theembodiment of FIGS. 1-7, through which an infusion tube 14, electrodelead wires 16 and a puller wire 18 extend. It is understood that, ifdesired, three lumen flexible tubing 24 may be used in which all lumensare diametrically aligned. A three off-axis lumen is preferred, however,because it provides superior strength. If bidirectional capability isdesired, the flexible tubing preferably comprises four off-axis lumens,and the puller wires 18 extending through diametrically opposed off-axislumens.

If desired, an electrically isolated ring electrode 58 may be mountedaround the distal end of the bridge tubing 22 as shown in FIG. 10. Insuch an embodiment, the lead wire for the ring electrode extends througha small hole in the bridge tubing 22 and across the exterior surface ofthe bridge tubing to the electrode 58. The lead wire is covered and thehole in the bridge tubing is filled with polyurethane resin or the like.More than one ring electrode may be carried by the bridge tubing or theflexible tubing.

In a further embodiment of the invention as shown in FIG. 11, the tipsection 12 carries an electromagnetic sensor 71 within the bridge tubing22. In this embodiment, the split tip electrode 20 may (as shown) or maynot comprise a cup electrode 36, as desired. The flexible tubing 24comprises at least three off-axis lumens, one for a cable 72 whichextends from the electromagnetic sensor 71 through the tip section 12,catheter body 11 and handle 13 to a connector for connection to a signalprocessing and imaging means. If desired, a circuit board can beprovided in the control handle. The sensor cable is connected to thecircuit board, which amplifies the signal received from theelectromagnetic sensor and transmits it to a computer in a formunderstandable by the computer. The other two lumens in the flexibletubing are for the infusion tube 14 and electrode lead wires (not shown)and for a puller wire (not shown). Again, if bidirectional capability isdesired, a fourth off-axis lumen in the flexible tubing would berequired for the second puller wire. Suitable electromagnetic sensorsand signal processing and imaging means are commercially available fromCordis Webster, Inc., and are described in U.S. Pat. Nos. 5,558,091,5,443,489, 5,480,422, 5,546,951, 5,568,809, and 5,391,199 andInternational Publication No. W095/02995 which are incorporated hereinby reference.

In this embodiment, the bridge tubing 22 is sufficiently long to allowthe infusion tube 14 to curve around the electromagnetic sensor 71 andinto the axial flange 51 of the cup electrode 36 into the centralirrigation channel 40 in the quad tip electrode.

It is to be understood that, with respect to the embodiments describedabove comprising a single puller wire, the catheter may comprise two ormore puller wires. Likewise, those embodiments described abovecomprising a pair of puller wires may, if desired, comprise only asingle puller wire. In an embodiment comprising a pair of puller wires,the flexible tubing 24 of the tip section 12 would preferably comprisefour off-axis lumens. The puller wires would preferably extend intodiametrically opposed off axis lumens and be anchored at the distal endof the flexible tubing as described above. Control handles formanipulating a pair of puller wires are well known. Preferred controlhandles are described in U.S. Pat. Nos. Re 34,502, 6,123,699, and6,120,476 to Webster, Jr., and U.S. patent application Ser. No.09/130,359 to Ponzi, all of which are incorporated herein by reference.

In the event that an omni-directional capability is desired, thecatheter may comprise three or more, and preferably four puller wires.In such an embodiment, the flexible tubing of the tip section wouldpreferably comprise four off-axis lumens, one in each quadrant, throughwhich the puller wires extend. A central lumen in the tip section wouldpreferably be provided to accommodate the infusion tube and electrodelead wires. Such an arrangement of lumens and puller wires and a controlhandle for manipulating the puller wires is disclosed in U.S. Pat. No.6,123,699 to Webster, Jr., which is incorporated herein by reference.

It is to be understood that any suitable means for controllablydeflecting the tip section in one or more may be used. Examples of othersuch means can be found in U.S. Pat. Nos. 5,656,030 to Hunjan, et al,5,195,968, 5,254,088, 5336,189, 5,531,686 to Lundquist, et al,5,273,535, 5,281,217, 5,370,678 to Edwards, et al, 5,358,478 toThompson, et al, 5,364,351 and 5,456,664 to Hemzelman, all of which areincorporated herein by reference.

It is understood that any construction providing a split tip electrodecomprising two or more electrode members may be used in this invention.The presence of a cup electrode, while preferred is not required.Likewise, the presence of a bridge tubing is preferred, but notrequired.

The ablation system comprises a first RF generator for generating a lowlevel RF impedance current and a second RF generator for generating anRF ablation current.

The low level RF impedance current may be any current having a powerlevel and frequency which allows detection of impedance without ablatingthe tissue or creating fibrillation, i.e., resulting in muscle capture,and preferably is at a frequency that tends to maximize the differencein impedance of tissue as compared with blood pool. A current of 2microamperes at 50 KHz is presently preferred.

The RF ablation current may be any current of sufficient power andfrequency to ablate myocardial tissue. An ablation current of from about0.25 to about 1.0 amperes or more, preferably about 0.5 to 0.75 amperes,at a frequency of from about 400 to about 700 KHz. and preferably about500 KHz. Higher power levels which result in deeper tissue lesions arepreferred as long as charring of the tissue surface can be avoided,e.g., by irrigation, and steam pops can be avoided.

The first and second RF generators may be two separate devices or, asshown in FIG. 1, may be combined into a single device. A presentlypreferred RF generator is the Stockert EP/Shuttle RF generator which iscommercially available from Cordis Webster, Inc. This generator includesmeans for generating a low level RF impedance current and an RF ablationcurrent. This RF generator also comprises means for receiving impedancesignals associated with each of the electrodes and filters forseparating these signals from the RF ablation currents in order toobtain clear impedance signals which may be recorded and/or displayed.This generator comprises displays for displaying the temperature andimpedance associated with an electrode member and a switch forselectively switching each of the displays from one electrode member toanother. The Stockert RF generator also comprises a display for showingthe amount of time which has elapsed during which RF current isdelivered to the ablating electrode member(s). It also has a safetyshut-off which shuts off the RF ablation current if the temperature orimpedance reaches a select predetermined level.

The monitor/display may comprise one or more suitable devices forreceiving signals indicative of the electrical activity of the heart,temperature of the electrode members and/or impedance associated witheach of the electrode members and for generating a display and/or recordof the received signals. As noted above, a display/monitor fordisplaying and/or recording the electrode member temperature andimpedance may be incorporated into the RF generator.

Alternatively, it may be built into the signal processor (discussedbelow). Independent stand alone displays may be used as desired.Suitable display/monitors for receiving and displaying electricalsignals received from a pair of electrode members as a bipolarelectrogram are commercially available, e.g., from Prucka Engineering.The metering pump may be any suitable metering pump capable of meteringan amount of from about 5 to about 60 cc/min., and preferably from about20 to about 40 cc/min., of a cooling fluid, e.g., a saline solution,into and through the infusion tube of the irrigated tip electrodecatheter. The Mark V Plus injection pump commercially available fromMedrad, Inc., or the Model 7100 volumetric infusion pump available fromIVAC Medical Systems are examples of suitable metering pumps for use inthe present invention.

The signal processor may be any programmable microprocessor or the like.The signal processor is electrically connected to the RF generator(s),the monitor/display and the pump, as well as the split tip catheter andthe reference electrodes. In a preferred embodiment, the signalprocessor is programmed to activate the RF generator to generate a lowlevel RF impedance current, e.g., about 2 microamps at about 50 KHz, andto transmit such current to each of the electrode members of the splittip electrode. The signal processor then receives signals indicative ofthe impedance associated with each electrode member, compares thoseimpedance signals and determines which electrode members are associatedwith the highest impedances. The signal processor then automaticallyactivates the RF generator to generate an RF ablation current and totransmit that current only to those electrode members determined to bein best contact with the myocardium.

During ablation, irrigation is used to keep the ablating electrodemembers cool. It also reduces the temperature of the tissue at thetissue-electrode interface. This allows for higher RF current to be usedresulting in deeper heating, i.e., the maximum tissue temperature wouldbe below the surface rather than at the surface, and a larger lesion.Irrigation, however, causes the temperature of the electrode to besignificantly less than that of the tissue at the tissue-electrodeinterface. For this reason, it is preferred to use intermittentirrigation so that, during the periods of no irrigation, the temperatureof the electrode members in contact with the tissue will rise and betterapproximate the tissue surface tissue temperature. During the periodswhen irrigation is stopped, it is preferred to also cease thetransmission of RF ablation current.

Accordingly, during the ablation procedure, the signal processoractivates the pump to pump saline solution or other cooling fluidthrough the catheter to cool the ablating electrode members. Preferably,the signal processor is programmed to activate the pump and RF generatorintermittently, i.e., to activate the pump and RF generator for a selectperiod of time, preferably from about 1 to about 10 seconds and morepreferably about 5 seconds, and then deactivate the pump and RFgenerator for another period of time, preferably from about 1 to about10 seconds, more preferably about 5 seconds and to continuously repeatthis cycle.

During ablation the signal processor receives signals from thethermocouple or thermistor associated with each of the electrodemembers, the signals being indications of the temperatures of theelectrode members, particularly at the end of a cycle wherein theirrigation and RF ablation current are shut off. These temperaturesignals are used to provide an estimate of the temperature of the tissueat the tissue electrode member interface. The signal processor isprogrammed to reduce or terminate the transmission of RF ablationcurrent when the temperature of an ablating electrode member reaches apredetermined level, e.g., 90° C.

During the ablation process, the signal processor preferably activatesthe RF generator to continue to transmit low level RF current signals toeach of the electrode members and receives signals indicating theimpedance through the blood and/or tissue during the ablation procedureassociated with each such electrode member. The signal processing unitdetermines the difference in impedance signals associated with thoseelectrode members in contact with the myocardium and those in contactonly with the blood pool. As the tissue heats up during the ablationprocedure, the impedance through the tissue decreases whereas theimpedance through the blood pool generally remains the same. Thedifference in impedance through the tissue as compared to through theblood, provides an indication of the maximum temperature of the tissue.The signal processor is therefore programmed to reduce or terminate theRF current being delivered to the selected electrode members when theimpedance associated with those electrode members drops to apredetermined level or when the difference between the impedanceassociated with the ablating electrode members in contact with themyocardium and the impedance associated with the electrode members incontact only with the blood pool reaches a predetermined level.

For a given set of parameters, e.g., power level, frequency, amount ofirrigation, cycling periods. angle of the ablation electrode, etc., thecorrelation between temperature and impedance may be determined, forexample, by in vivo testing on dogs. A preferred test comprises a seriesof ablations under controlled conditions using, e.g., dog thigh muscle.The temperature of the muscle at various depths can be measured using afluoroptic thermal probe, e.g., Luxtron model 3000 or the like, andcorrelated against measured impedance. A testing procedure suitable foruse in the present invention is described in “Comparison of In VivoTissue Temperature Profile and Lesion Geometry for RadiofrequencyAblation With a Saline-Irrigated Electrode Versus Temperature Control ina Canine Thigh Muscle Preparation” Circulation, 1995; 91:2264-2273, andInverse Relationship Between Electrode Size and Lesion Size DuringRadiofrequency Ablation With Active Electrode Cooling, Circulation,1998; 98:458-465, which are incorporated herein by reference. From suchtesting, a correlation between measured impedance, and maximum tissuetemperature can be obtained. The signal processor is programmed toreduce or shut down the transmission of RF ablation current to theelectrodes if the maximum tissue temperature of the tissue exceeds apredetermined level, e.g., 90° C., as determined by the impedancemeasurements.

It is understood that information regarding the tissue temperature canbe obtained by measuring the impedance associated with only thoseelectrode members being used for ablation. However, monitoring theimpedance associated with each ablating electrode member, i.e., those incontact with the myocardium and those non-ablating electrode members incontact with only the blood pool, is preferred. In such a system,changes in impedance due to factors other than the increasing tissuetemperature would not affect, or at least would have minimal effect, onthe impedance-temperature correlation.

Monitoring the impedance associated with each of the electrode membersduring ablation also allows the electrophysiologist to continue tomonitor which electrode members are in contact with the myocardium andhence, whether the catheter tip has rolled or rotated during ablation.

It is understood that the signal processor need not accomplish all ofthe functions described above. Indeed, many, if not all, of thefunctions described above with respect to the signal processor may beaccomplished manually, e.g., by the electrocardiologist. For example,the electrocardiologist may analyze the various impedance measurementsdescribed above and then manually select, e.g., by a dial on the RFgenerator, the electrode members in best contact with the myocardium toreceive RF ablation current. The electrocardiologist may then manuallyactivate the RF generator to generate that RF ablation current.Likewise, the electrocardiologist may monitor temperature and impedancereadings during ablation and manually reduce or discontinue RF currentif the temperature and/or impedance readings indicate excessive tissuetemperature.

In use, the split tip electrode catheter is inserted into the heart andthe split tip electrode is contacted with heart tissue at variouslocations in the heart to map the electrical activity of the heart. Ateach such location, electrode members in contact with the myocardiumsense electrical signals propagating through the heart. By means of theelectrode arrangement of the split tip electrode at least two electrodeswill be in contact with the myocardium. If the tip is lying parallel tothe tissue or at an angle of less than about 30° to about 45°, the ringelectrode is also in contact with or close to the myocardium. Thisenables two orthogonal electrical signals, each with a voltage level andpolarity, to be sensed, which provides sufficient data to determineexcitation wave front direction. If the split tip electrode is orientedperpendicular to the endocardial surface, all four of the hemisphericalelectrode members of the split tip electrode would be in contact withthe myocardium. In this orientation, at least two orthogonalelectrograms can be obtained from the four hemispherical electrodemembers. This also provides sufficient data to determine a wave frontdirection.

Once the endocardium is sufficiently mapped to determine the location tobe ablated, the electrode members of the split tip electrode in goodcontact with the endocardial tissue are determined. This is done bydelivering a low level RF current, e.g., about 2 microamperes at about50 KHz, to each electrode member of the split tip electrode. Theimpedance between each such member and a reference electrode(s) such asa skin patch electrode(s) on the patient is then measured. The impedanceassociated with those electrode members in contact with the tissue willbe higher than those that are surrounded by the blood pool. Accordingly,the electrode member(s) with the highest impedance will be the one(s) inbest contact with the endocardial tissue.

Once the ablation site has been located and the electrode members mostfully in contact with the endocardial tissue have been identified, RFcurrent for ablation is selectively delivered to those electrodemembers. RF energy is delivered at a power level and at a frequency andfor a time sufficient to kill enough heart tissue to interrupt theaberrant electrical pathway. A time generally in the range of from about30 seconds to about 2 minutes is typical.

During ablation, it is important to prevent charring of the surface ofthe myocardium and overheating of the deep heart tissue which couldresult in “steam pops.” Accordingly, temperature of the tissue ispreferably monitored in two ways. First, each electrode member of thesplit tip electrode has a temperature sensor, e.g., a thermocouple orthermistor, associated with it. The temperature of the endocardium atthe interface with the electrode, is estimated by monitoring thetemperature of the electrode members in contact with the endocardium.Preferably, the electrode members are irrigated during ablation. Thisallows the temperature of the electrode members to be controlled so thata greater amount of RF current can be delivered through the electrodemembers without excessive temperature rise. Irrigation, however, createsa greater differential between electrode temperature and tissueinterface temperature. As a result, irrigation and the delivery of RFablation current is preferably intermittent, with periods of irrigationand RF ablation current separated by periods of no irrigation and noablation current. During the latter periods, the temperature of theelectrode members in contact with the tissue will rise to a temperaturemore indicative of the tissue interface temperature.

As a result of the greater RF current which can be used with anirrigated ablation electrode, the tissue below the interface, whichtends to heat more slowly, will rise to a temperature greater than thatat the interface. This temperature is estimated by monitoring theimpedance associated with each of the electrode members. As the tissueheats, the tissue impedance decreases whereas the impedance of the bloodpool remains essentially constant. Hence, by monitoring the impedanceassociated with the electrode members in contact with the tissue andcomparing the decrease in that impedance to the impedance associatedwith non-tissue contacting electrode members and correlating thatimpedance difference with temperature (as described above), the maximumtissue temperature can be estimated and excessive rise in tissuetemperature can be avoided.

If desired, the split tip electrode catheter may be equipped with anelectromagnetic sensor positioned proximal to the split tip electrode.The electromagnetic sensor in combination with suitable signalprocessing and imaging means, as described above, allow for thegeneration of a three-dimensional image of the interior contours of theheart chamber in which the distal tip of the catheter is located and tomonitor the location of the split tip electrode within that heartchamber.

The preceding description has been presented with reference to presentlypreferred embodiments of the invention. Workers skilled in the art andtechnology to which this invention pertains will appreciate thatalterations and changes in the described structure may be practicedwithout meaningfully departing from the principal, spirit and scope ofthis invention. Accordingly, the foregoing description should not beread as pertaining only to the precise structures described andillustrated in the accompanying drawings, but rather should be readconsistent with and as support to the following claims which are to havetheir fullest and fair scope.

What is claimed is:
 1. A system for ablating tissue and estimating themaximum sub-surface temperature of said tissue comprising: a split tipelectrode catheter comprising: an elongated catheter body having aproximal and distal end and at least one lumen extending therethrough; acatheter tip section at the distal end of the catheter body comprising asection of flexible tubing having proximal and distal ends and at leastone lumen therethrough, the proximal end of the flexible tubing beingfixedly attached to the distal end of the catheter body, said cathetertip section further comprising a split tip electrode which comprises atleast two electrically isolated electrode members fixedly attached tothe distal end of the flexible tubing, each electrode member comprisingat least one irrigation passage; at least one indifferent electrodeattachable to the exterior of a patient; means for generating a lowlevel RF impedance current and for transmitting said impedance currentto each electrode member of the split tip electrode; means forgenerating an RF ablation current and for transmitting said ablationcurrent to each electrode member of the split tip electrode; means forelectrically connecting each electrode member of the split tip electrodeto the low level RF impedance current generating means and the RFablation current generating means; means for receiving electricalsignals indicative of the impedance between each electrode member andthe at least one indifferent electrode during the transmission of RFablation current and for separating such impedance indicating signalsfrom other received signals; and a programmable signal processor,electrically connected to the RF impedance current generating means, theRF ablation current generating means, each of the electrode members ofthe split tip electrode, the at least one indifferent electrode and thesignal receiving means, said signal processor being programmed tosimultaneously activate the RF impedance current generating means andthe RF ablation current generating means to transmit RF impedancecurrent and RF ablation current and to estimate the maximum subsurfacetemperature based on the received electrical impedance-indicatingsignals.
 2. A system as claimed in claim 1 wherein the signal processoris further programmed to deactivate the RF ablation current generatingmeans if the estimated maximum subsurface temperature reaches apredetermined temperature.
 3. A system as claimed in claim 1 furthercomprising means for controllably deflecting the tip section in at leastone direction.
 4. A system as claimed in claim 1 wherein the split tipelectrode comprises four electrode members.
 5. A system as claimed inclaim 4 wherein the four electrode members are generally the same andare arranged symmetrically around a longitudinal axis of the tipsection.
 6. A system as claimed in claim 5 further comprising a ringelectrode adjacent proximally to the four electrode members.
 7. A systemas claimed in claim 1 wherein the low level RF impedance current hasampere level and frequency that allows detection of impedance withoutablating tissue in contact with the split tip electrode and withoutcreating fibrillation of a patient's heart.
 8. A system as claimed inclaim 7 wherein the ampere level and frequency selected are ones withwhich there is a measurable difference in impedance of tissue ascompared to the blood pool.
 9. A system as claimed in claim 1 whereinthe RF impedance current is about 2 microamperes at about 50 KHz.
 10. Asystem as claimed in claim 1 wherein each of said at least two electrodemembers of the split tip electrode comprises at least one irrigationpassage and wherein the system further comprises a metering pump forpumping a cooling fluid through the catheter and the at least oneirrigation passage in each electrode member to cool the electrodemembers during ablation.
 11. A system as claimed in claim 10 wherein thesignal processor is further programmed to activate the pump to pumpcooling fluid through the catheter to cool the electrode members whilean RF ablation current is being transmitted to one or more electrodemembers.
 12. A system for ablating tissue and estimating the surface andmaximum subsurface temperature of said tissue comprising: a split tipelectrode catheter comprising: an elongated catheter body having aproximal and distal end and at least one lumen extending therethrough; acatheter tip section at the distal end of the catheter body comprising asection of flexible tubing having proximal and distal ends and at leastone lumen therethrough, the proximal end of the flexible tubing beingfixedly attached to the distal end of the catheter body, said cathetertip section further comprising a split tip electrode which comprises atleast two electrically isolated electrode members fixedly attached tothe distal end of the flexible tubing each electrode member comprisingat least one irrigation passage; at least one indifferent electrodeattachable to the exterior of a patient; means for generating a lowlevel RF impedance current and for transmitting said impedance currentto each electrode member of the split tip electrode; means forgenerating an RF ablation current and for transmitting said ablationcurrent to each electrode member of the split tip electrode; means forelectrically connecting each electrode member of the split tip electrodeto the low level RF impedance current generating means and the RFablation current generating means; means, including a pump, for infusinga cooling fluid through the catheter and each of the electrode membersto cool the electrode members during ablation; temperature sensingmeans, attached to each electrode member of the split tip electrode, forgenerating a signal indicative of the temperature of the electrodemember to which it is attached; means for receiving electrical signalsindicative of the impedance between each electrode member and the atleast one indifferent electrode during the transmission of RF ablationcurrent and for separating such impedance indicating signals from othersignals received; and signal processing means, electrically connected tothe RF impedance current generating means, the RF ablation currentgenerating means, the pump of the infusing means, each of the electrodemembers of the split tip electrode, the at least one indifferentelectrode, the temperature sensing means and the signal receiving means,for activating the RF impedance generating means to transmit animpedance current to each of the electrode members of the split tipelectrode, for receiving electrical signals indicative of the impedancebetween each of the electrode members of the split tip electrode and theat least one indifferent electrode and for identifying the electrodemember(s) associated with the highest impedance signal(s) received asthe electrode member(s) in contact with tissue, and thereafter forintermittently activating the pump of the infusing means and the RFablation current generating means so that there are ON periods whereincooling fluid is delivered to the electrode members and RF ablationcurrent is transmitted to the identified tissue-contacting electrodemembers and OFF periods wherein no cooling fluid is delivered to theelectrode members and no RF ablation energy is transmitted to theidentified tissue-contacting electrode members for estimating thesurface temperature of tissue in contact with the electrode members fromthe signals received from the temperature sensing means during the OFFperiods, for activating the RF impedance current generating means totransmit impedance current to the electrode members of the split tipelectrode during at least the ON periods and for estimating the maximumsubsurface temperature of tissue heated by the RF ablation current fromthe received impedance-indicating signals.
 13. A system as claimed inclaim 12 further comprising means for controllably deflecting the tipsection in at least one direction.
 14. A system as claimed in claim 12wherein the split tip electrode comprises four electrode members.
 15. Asystem as claimed in claim 14 wherein the four electrode members aregenerally the same and are arranged symmetrically around a longitudinalaxis of the tip section.
 16. A system as claimed in claim 15 furthercomprising a ring electrode adjacent proximally to the four electrodemembers.
 17. A system as claimed in claim 12 wherein the low level RFimpedance current has ampere level and frequency that allows detectionof impedance without ablating tissue in contact with the split tipelectrode and without creating fibrillation of a patient's heart.
 18. Asystem as claimed in claim 17 wherein the ampere level and frequencyselected are ones with which there is a measurable difference inimpedance of tissue as compared to the blood pool.
 19. A system asclaimed in claim 18 wherein the RF impedance current is about 2microamperes at about 50 KHz.
 20. A system as claimed in claim 12wherein the signal processing means is further programmed to deactivatethe RF ablation current generating means if the estimated maximumsubsurface temperature reaches a predetermined temperature.
 21. A methodfor ablating tissue and for estimating the increase in subsurfacetemperature of tissue at an ablation site during an ablation procedurecomprising: providing a split tip electrode catheter comprising: anelongated catheter body having a proximal and distal end and at leastone lumen extending therethrough; a catheter tip section at the distalend of the catheter body comprising a section of flexible tubing havingproximal and distal ends and at least one lumen therethrough, theproximal end of the flexible tubing being fixedly attached to the distalend of the catheter body, said catheter tip section further comprising asplit tip electrode which comprises at least two electrically isolatedelectrode members fixedly attached to the distal end of the flexibletubing; introducing the catheter into a lumen in the body of a patientand positioning the catheter so that at least one electrode member ofthe split tip electrode is in contact with the tissue to be ablated andat least one other electrode member of the split tip electrode is onlyin contact with the blood pool; positioning at least one indifferentelectrode on the patient; transmitting an RF ablation current to atleast one tissue-contacting electrode member of the split tip electrode;transmitting a low level RF impedance current through the at least onetissue contacting electrode member of the split tip electrode duringtransmission of RF ablation current to the at least onetissue-contacting electrode member; monitoring the impedance between theat least one tissue-contacting electrode member and the at least oneindifferent electrode and the impedance between the at least onenon-tissue-contacting electrode member and the indifferent electrode;and estimating the increase in subsurface tissue temperature from thechange in the difference between the impedance associated with the atleast one tissue-contacting electrode member and the impedanceassociated with the at least one non-tissue-contacting electrode member.22. A method as claimed in claim 21 further comprising reducing orterminating the transmission of RF ablation current to the at least onetissue-contacting electrode member of the split tip electrode when thechange in measured impedance reaches a predetermined level whichindicates that a predetermined maximum subsurface temperature has beenreached.
 23. A method as claimed in claim 21 wherein the catheterfurther comprises means for controllably deflecting the tip section inat least one direction.
 24. A method as claimed in claim 21 wherein thetip electrode further comprises four electrically isolated electrodemembers.
 25. A method as claimed in claim 24 wherein the four electrodemembers are generally the same and are arranged symmetrically around alongitudinal axis of the tip section.
 26. A method as claimed in claim25 wherein the catheter further comprises a ring electrode adjacentproximally to the four electrically isolated electrode members.
 27. Amethod as claimed in claim 21 wherein the low level RF impedance currenthas ampere level and frequency that allows detection of impedancewithout ablating tissue in contact with the tip electrode and withoutcreating fibrillation of a patient's heart.
 28. A method as claimed inclaim 27 wherein the ampere level and frequency selected are ones withwhich there is a measurable difference in impedance of tissue ascompared to the blood pool.
 29. A method as claimed in claim 28 whereinthe RF impedance current is about 2 microamperes at about 50 KHz.
 30. Amethod as claimed in claim 21 wherein the tip electrode comprises atleast one irrigation passage and wherein cooling fluid is passed throughthe catheter and the least one irrigation passage to cool the tipelectrode while an RF ablation current is being transmitted to the tipelectrode.